相槌 refers to a custom in Japanese culture wherein it is considered polite and necessary to indicate while listening that you are in fact listening. This is usually done through terse comments (such as "I see"), affirmative grunts and other noises, and gesticulations, such as a slight nodding of the head. While it is common to do this in the U.S., it is not as clearly defined a custom. Thus, it is not generally considered impolite when a person doesn't make 相槌, and there no single word to describe it.